(1) Field of the Invention
This invention relates to sheet handling apparatus in general, and more specifically, the invention relates to an improved electronic sheet registration apparatus for registering original documents on the document platen of a copier/duplicator machine.
(2) Description of the Prior Art
The use of registration mechanism for registering document sheets on the document platen of a copier/duplicator machine is well known in the prior art. Prior art registration mechanisms may be manual or automatic.
In the manual type prior art devices, an operator places a document to be copied at the registration zone of the document platen. Usually, the registration zones are identified by indicia disposed along the sides of the document platen.
The automatic registration mechanisms usually coact with automatic document feeders (ADF) or semiautomatic document feeders (SADF) to position the document. U.S. Pat. No. 4,243,316 is an example of the prior art registration mechanisms. The registration mechanism is used to register sheets on the document platen of a copier/duplicator. A sheet feeder of the vacuum transport type is mounted by a hinge to the frame of the copier/duplicator. The sheet feeder is disposed above the document platen and is movable between an elevated position and a lowered position. When the feeder is in the elevated position, a document sheet can be manually placed against a registration mechanism to properly locate the sheet for exposure and copying. When the feeder is moved into a lowered position, a sheet from a hopper or tray in the top of the feeder is removed by the feeder and is advanced across the platen surface into engagement with the registration mechanism prior to exposure and copying.
The registration mechanism is of the mechanical type and consists of an elongated gate member or bar pivotally mounted to the frame of the copier/duplicator. The elongated gate runs transversely to the document platen and parallel to one edge of said platen. The elongated gate member has a first and a second surface that are disposed at an obtuse angle and meet along the edge. In one of its pivotal positions (called the lower position), the obtuse angle of the elongated gate member mates with the edge to register a sheet. In the other pivotal position (called the raised position), the elongated gate member is raised above the platen and sheets can be transported freely on the platen.
With the bar in the raised position, a sheet can be driven in a first direction past the registration mechanism, inverted and then returned to the platen so that the second side of the sheet can be copied. The procedure is often referred to as duplex copying.
Other types of prior art registration gates consist of a mechanical gate which descends or ascends into the path of the sheet to be registered. If the registration gate is mounted to the document feeder, it usually descends in a vertical orientation relative to the exposure platen and stops a sheet at the proper registration zone. Alternately, when the registration gate is mounted on the copier frame, it ascends upwardly to stop the sheet. As before, the gate is disposed vertically with respect to the document platen. Examples of the vertical type registration gates are disclosed in U.S. Pat. Nos. 4,176,945 and 4,179,215.
Yet another type of prior art registration mechanism is disclosed in an article entitled "Document Feeder without Registration Gate" by Charles J. Mahler and published in Volume 2, Number 3, May/June 1977 (pg. 49) of the Xerox Disclosure Journal. In the article, a document to be registered on the platen of a copier is preregistered against a pivotal stop. The document is then fed in a timed relation onto the platen. A segmented friction drive system controls a platen transport system so that the sheet is driven a given distance after preregistration.
Although the prior art mechanical gates work satisfactorily for its intended purpose, such registration mechanisms tend to be relatively slow and tend to damage the edge of the document. The edge damage to sheets is the result of the sheets impacting on the gate member. The damage increases as the speed with which the sheets are transported increases.
The slowness of the mechanical gates stem from the fact that there is a lag time between the issuance of a command to position and/or remove the gate from its registration position at the document glass. For high speed copiers it is necessary-that a sheet be positioned and removed from the document platen in a relatively short period of time. As such, mechanical gates tend to reduce the throughput of the system and are not suitable for use in high performance machines. U.S. Pat. No. 4,066,255 is another type of prior art registration apparatus which does not require a gate for registering sheets on the document platen. The apparatus consists of a switch disposed between a sheet feeder and a platen transport mechanism. The sheet feeder delivers sheets in seriatim to the platen transport mechanism. As sheets are dispensed by the feeder, the sheets trip the switch. Tripping of the switch activates the platen transport and deactivates the feeder. The platen transport is controlled to transport the sheet a predetermined distance to the registration position.
Vol. 3, Number 2, March/April 1978 (pgs. 123-125) of the Xerox Disclosure Journal describes a document registration system in which the leading edge of a document is transported past a switch or photo-optical lead edge sensor. The switch or photo-optical sensor is disposed on the downstream side of a sheet platen registration zone in the direction of travel. The sheet is then reversed for a fixed short distance equal to the fixed distance between the sensor and the registration zone.
Admittedly, the latter nonmechanical registration devices are an improvement over the prior art mechanical registration devices. However, the prior art nonmechanical devices do suffer from a few drawbacks. By way of example, the prior art nonmechanical registration devices do not contemplate bidirectional registration of a sheet. As such, the prior art devices are not suitable for registering documents in a copier operating in a duplex mode. As was stated previously, duplex operation requires that both sides of the sheet must be copied. Similarly, the prior art nonmechanical registration devices require that the sensor be disposed at a fixed distance from the registration zone or line on the document platen. As such, once a registration zone is established relative to a sensor, it is almost impossible to register a document at any other point on the document platen. This drawback tends to unduly restrict the use of document feeders to position a sheet on the document platen. In fact, this limitation or defect is also a characteristic of the prior art mechanical gates.